Field of the Invention
The present invention relates to capping layers for optical elements, e.g. multilayer mirrors, for use with extreme ultraviolet (EUV) radiation. More particularly, the invention relates to the use of capping layers on optical elements in lithographic projection apparatus comprising:                an illumination system for supplying a projection beam of radiation;        a first object table provided with a mask holder for holding a mask;        a second object table provided with a substrate holder for holding a substrate; and        a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.        
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask-table” and the “substrate table”, respectively.
In the present document, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term “vertical” (Z) used herein is intended to refer to the direction normal to the substrate or mask surface or parallel to the optical axis of an optical system, rather than implying any particular orientation of the apparatus. Similarly, the term “horizontal” refers to a direction parallel to the substrate or mask surface or perpendicular to the optical axis, and thus normal to the “vertical” direction.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire a reticle pattern onto the die at ones; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M generally <1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in “Synchrotron radiation sources and condensers for projection x-ray lithography”, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
Optical elements for use in the EUV spectral region, e.g. multilayered thin film reflectors, are especialy sensitive to physical and chemical damage which can significantly reduce their reflectivity and optical quality. Reflectivities at these wavelengths are already low compared to reflectors at longer wavelengths which is a particular problem since a typical EUV lithographic system may have nine mirrors; two in the illumination optics, six in the imaging optics plus the reflecting reticle. It is therefore evident that even a “small” decrease of 1-2% in the peak reflectivity of a single mirror will cause a significant light throughput reduction in the optical system.
A further problem is that some sources of EUV radiation, e.g. plasma based sources, are “dirty” in that they also emit significant quantities of fast ions and other particles which can damage otical elements in the illumination system.
Proposals to reduce these problems have involved maintaining the optical systems at very high vacuum, with particularly stringent requirements on the partial pressures of hydrocarbons which may be adsorbed onto the optical elements and then cracked by the EUV radiation to leave opaque carbon films.